System error! your system is infected with dangerous virus! keeps popping up!!!?

system error! your system is infected with dangerous virus! Note: strongly recommended to install antispyware programme to clean your system and aviod total crash of your computer! Click OK to download the antispyware(recommended) this error thing appears when i go into my computer or my documents.. please... show more system error!
your system is infected with dangerous virus!
Note: strongly recommended to install antispyware programme to clean your system and aviod total crash of your computer!
Click OK to download the antispyware(recommended)

this error thing appears when i go into my computer or my documents.. please tell me how do i get rid of it. am i really infected with virus?? i clicked ok to download the spyware thing but =.= they scan it for u free and require u to pay to remove the virus.. help me pls thank you
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